The present invention relates generally to a method and apparatus having a work table along which a strip of sheet material is indexed and relates more particularly to improved means for insuring that the sheet material is indexed in precisely controlled increments.
Prior art apparatus having a work table defining a support surface over which sheet material is spread while an instrument or tool which is supported on a carriage above the support surface performs a work operation on the material is disclosed in U.S. Pat. No. 4,091,980 having the same inventor as the present invention. Examples of instruments associated with such a work table include plotting pens or styluses, light heads, tracking heads and cutting or drilling tools. Typically, the carriage is movable over the support surface in one coordinate direction parallel to the support surface while the instrument, mounted on the carriage, is movable relative to the carriage and in another coordinate direction. Composite movements of the carriage and instrument allow the instrument to be translated to any coordinate position over the region of the support surface traversed by the carriage. Accurate positioning of the carriage and the instrument supported thereby may be achieved by numerical controls which may operate either from an on-line data generator or from previously programmed data.
The prior art apparatus also includes means for indexing long strips of the sheet material over the work table which is relatively short compared to the length of the strip. The indexing means advances or shifts the material across the surface of the table so that the instrument may operate on successive sections of the material.
The difficulty in working on successive sections of a strip of material is that movement of each section of the strip into working position must be precisely controlled to permit generation of continuous patterns which extend between adjacent sections of the strip. In a high resolution plotting system, for example, a series of plotted lines may extend continuously over several successive sections of a strip which is substantially longer than the plotting table. The strip indexing operation, unless accurately controlled, may cause discontinuies in the plot and results in plotted lines that are out of registration on adjacent sections.
If the indexing apparatus shown in U.S. Pat. No. 4,091,980 is not used, the amount of error in an indexing operation is typically dependant upon precise operation of belts, clutches or drive motors. However, if the belts stretch or clutches slip or if the motor shafts continue to rotate due to inertia effects after power to the motor is shut off, error in the indexing operation will result.
It is, accordingly, a general object of the present invention to provide an alternative to the apparatus in patent cited above, an apparatus and method for precisely controlling the amount of sheet material advancement across the work table. The invention obviates predictable error-producing phenomena associated with material indexing means of the prior art.